In the field of networks, there are instances when it is desired or required that data terminal equipment (DTE) be able to draw power from the same generic cabling as that used for data transmission. DTE devices may include telephones, Voice over Internet Protocol (VoIP or IP or Ethernet) telephones, network access devices, computers, and the like. Such a power scheme is known as inline or phantom power or power via a media dependent interface (MDI). Various example power via MDI schemes exist. These include a proprietary scheme from Cisco Systems Inc (Cisco) and a standard scheme from the Institute of Electrical and Electronics Engineers (IEEE). The proprietary scheme from Cisco will be referred to as Cisco legacy power. The IEEE scheme is known as IEEE 802.3af standard power and will be referred to as IEEE standard power. Although these two inline power schemes have some aspects in common, they are not entirely compatible with one another.
Turning first to FIG. 1, a schematic diagram of a network power system 10 having power source equipment (PSE) 12, a plurality of powered devices (PD) 14A-N, and a plurality of corresponding transmission media 16A-N is shown. The plurality of transmission media 16A-N are connected to the corresponding power source equipment 12 and powered devices 14A-N through a power interface at each end of the transmission media. Each transmission media may contain a plurality of conductors. For example, the current Ethernet standard is a minimum of two twisted-pair cables for a total of four conductors. The length and routing of the transmission media will depend on the circumstances and the applicable communications protocol. The number and location of each of the plurality of powered devices 14A-N will depend on the circumstances. The power source equipment 12 may also be variously located based on the circumstances. The power source equipment 12 may be at one end of the network or it might be somewhere between the ends of the network. The multiple power interfaces of the power source equipment 12 are often referred to as ports. The number of ports will vary. Typically, one port is connected to one powered device. However, it is also possible that multiple powered devices might be connected together to one port or multiple ports might be connected to one powered device. The multiple transmission media are often referred to as links between the power source equipment and the various powered devices. Each link includes at least two data and power signal paths with one for transmitting and one for receiving. In order for the network power system 10 to operate correctly, the power source equipment 12 and the plurality of powered devices 14A-N must be compatible with one another.